Fiber-reinforced rubber compound useful in PDM stators

ABSTRACT

A rubber compound for use in a stator. The stator may be deployed in a positive displacement motor. The rubber compound includes a fiber reinforcement, wherein fibers in the fiber reinforcement create a grain direction in which “with the grain” is generally orthogonal to “across the grain”. In some embodiments, the rubber compound has a first value for 25% tensile Modulus across the grain and a second value for 25% tensile Modulus with the grain, wherein the first value is at least 10% lower than the second value. In such embodiments, the fiber reinforcement may further include a fiber loading of greater than 1.0 phr of fibers. In such embodiments, the rubber compound may further have a 25% tensile Modulus of greater than 400 psi across the grain and a 50% tensile Modulus of greater than 700 psi across the grain.

RELATED APPLICATIONS

This application is a continuation of commonly-invented andcommonly-assigned U.S. nonprovisional patent application Ser. No.17/318,785 filed May 12, 2021 (U.S. Pat. No. 11,542,944). Ser. No.17/318,785 is a continuation of commonly-invented and commonly-assignedU.S. nonprovisional patent application Ser. No. 16/258,826 filed Jan.28, 2019 (U.S. Pat. No. 11,015,603). Ser. No. 16/258,826 is acontinuation of commonly-invented and commonly-assigned U.S.nonprovisional patent application Ser. No. 15/292,798 filed Oct. 13,2016 (U.S. Pat. No. 10,215,176). Ser. No. 15/292,798 claims the benefitof, and priority to, commonly-invented and commonly-assigned U.S.provisional patent application Ser. No. 62/240,876, filed Oct. 13, 2015.The entire disclosures of Ser. Nos. 17/318,785, 16/258,826, 15/292,798and 62/240,876 are incorporated herein by reference.

FIELD OF THE DISCLOSURE

This disclosure is directed generally to fiber reinforcement technologyfor optimizing the properties of rubber used, for example, in themanufacture of stators in positive displacement motors (“PDMs”).

BACKGROUND OF THE DISCLOSED TECHNOLOGY

The fiber reinforcement of rubber technology described in thisdisclosure optimizes the properties of the rubber, and thus is useful inmany industrial or commercial applications. The application of thetechnology to optimize the properties of rubber used in PDM stators willbe used in this disclosure to illustrate one such application.

This disclosure describes a range of optimized fiber-reinforced rubbercompositions, and methods of making them, for use in the statorinjection process. During the rubber injection process to make stators,the rubber is injected though a mold that requires the rubber to flowthrough a geometry with a very high length to cross section ratio.Typical stator tube geometries may have lengths of 120″ to 300″ for tubediameters of 4.75″ and larger. Stator tubes in the 2″ to 4″ diameterrange have typical lengths of 60″ to 150″ and stator tubes in the 1.5″to 2″ have typical lengths of 50″ to 100″. As a result of the injectionflow process to achieve these geometries, significant grain direction atthe rubber molecular level is established in the lobes of the stator.The establishment of a grain in the flow direction is unavoidable,creating undesirable anisotropy in the rubber when cured.

Rubber anisotropy in the stator causes the material properties of thefinal rubber product to be different in the cylindrically transversecross-section direction of the stator than in the cylindricallongitudinal direction. In fact, rubber flow during injection is moreaccurately in a helical pathway flowing in a generally longitudinaldirection. Thus the rubber chain molecule grain follows a helicalpathway, although performance metrics of the stator look more closely inthe cylindrical longitudinal direction and the cylindrical transversecross-section.

Persons of ordinary skill in this art will understand that, consistentwith applicable standards such as ASTM D412, terms such as “Young'sModulus”, “Modulus of elasticity”, “tensile Modulus”, or just “Modulus”(as used in this disclosure) are interchangeable to describe a parameterrepresenting the general propensity of a material to deform (elongate)under a tensile stress load. The value of Modulus for a particularmaterial is generally measured in Pascals, and quantifies the material'spropensity to deform under tensile load. The value of Modulus thuspredicts an elongation in the material (or a “strain” in the material)for a given tensile stress load. Conversely, the value of Moduluspredicts the tensile stress required to be applied to the material toachieve a certain elongation (or “strain”). Thus, by way of example andagain consistent with ASTM D412, the term “25% tensile Modulus” or “25%Modulus” as used in this disclosure refers to the tensile stress appliedto a material (or seen in a material) at 25% elongation, “50% tensileModulus” or “50% Modulus” refers to the tensile stress applied or seenat 50% elongation, “100% tensile Modulus” or “100% Modulus” at 100%elongation, and so on. Modulus of elasticity (or simply “Modulus” asused in this disclosure) is one important material performance propertyof rubber in PDM stators. Modulus is also a somewhat reliable indicatorof other desirable material properties, in that higher Modulus willnormally indicate higher tensile strength and crack resistance. Withoutsome sort of reinforcement, the rubber anisotropy inevitably caused byinjection molding in stator manufacturing causes the cured rubber toexhibit lower Modulus in the cylindrical transverse cross-sectiondirection (“against the grain” or “across the grain”) versus in thecylindrical longitudinal direction (“with the grain”). Low Modulus inthe transverse direction leads to premature breakdown and “chunking” ofthe rubber under cyclic operational loads in a PDM.

Elongate fibers introduced into the rubber strengthen the rubbercomposite, and improve material properties such as crack resistance.When added to rubber, small amounts of fiber can significantly improvethe life of components by acting to distribute stress across thecomponent more effectively. This is particularly effective as thecomponent weakens during cyclic loading. Fibers distribute and dissipateenergy at the crack tip of any flaw initiation site, thereby slowing thecrack initiation and propagation stage of fatigue failures.

Unfortunately, however, elongate fibers within a rubber composite aresusceptible to the same grain alignment during manufacture as theunderlying rubber chain molecules. Thus conventional fiber-reinforcedrubber composites do little to address loss in transverse Modulus, forexample, due to the underlying rubber molecule chain anisotropy. Thereality is that when elongate fibers are added to the rubbercomposition, the fibers also tend to align substantially with the grain,i.e. in the flow direction of the helical path of the lobe geometry. Themost significant changes in material physical properties enabled by thefibers will be aligned with this helical path and substantially alongthe length of the stator. In the transverse cross-sectional direction,the material properties will tend to change less. In order to enhancetransverse material properties such as transverse Modulus, therefore, itbecomes desirable to load the fiber content of the rubber as high aspossible, and/or to use high strength fibers as much as possible.However, high fiber load and/or use of high strength fibers may causeother performance issues with the rubber composition, both in chemistryand in material properties. In particular, high fiber load and/or use ofhigh strength fibers is known to reduce flexibility and crackingresistance in some applications, especially at lower temperatures. Therehas been a longfelt but unsolved need in the PDM stator art for rubbercomposite products that carry a high fiber load and/or use high strengthfibers, and that have also maintained serviceable chemistry or materialproperties in other aspects.

U.S. Pat. No. 6,358,171 to Whitfield discloses fiber loading of a rubbercomposite in tension belt applications (such as automotive timingbelts). In column 3, line 65 through column 4, line 9, Whitfield positsthat the dispersed fibers inhibit crack propagation and growth in thebelt rubber during operational loads, thereby improving performance ofthe belt at both high and low temperatures. Whitfield further disclosesthat the fibers increase the shear strength of the teeth and thusprovide a higher load-carrying capability than a similar belt madewithout fiber reinforcement.

While instructive on the operational benefits of fiber-reinforced rubbercomposites generally, Whitfield does not address the anisotropy problemin the PDM stator art identified above, namely achievement ofserviceably high Modulus in the transverse cross-section direction(“against the grain”) when the manufacturing process necessarily createssubstantial fiber alignment in the longitudinal direction (“with thegrain”). As can be seen from the Figures in Whitfield, the fibers arealigned in the direction of travel of the belt. Because the belt isretained by pulleys in operation, the belt undergoes comparativelylittle load in the transverse direction (“against the grain”).

The rubber composition disclosed by Whitfield nonetheless forms aserviceable starting point from which to develop a new rubbercomposition, as disclosed in this application. The modified rubbercomposition will address the problems in the PDM stator art describedabove.

U.S. Published Patent Application 2015/0022051 to Meng et al. (“Meng”)discloses a fiber-reinforced rubber composite material for use in PDMstators. In paragraph 0008, Meng identifies reasons why the prior arthas had difficulty deploying such fiber-reinforced rubber composites ininjection molding manufacturing process (such as are generally used inPDM stator manufacturing), and further identifies poor fiber dispersionthroughout the composite matrix as a primary culprit. Meng improvesdispersion via use of a solid “fiber dispersion compound”, such asamorphous silicon dioxide, admixed with the fibers into the rubber.Although Meng confines its disclosed embodiments to use of such a solidfiber dispersion compound, Meng defines “fiber dispersion compound” toinclude solid agents, liquid agents or a combination of both. Mengdiscloses use of a fairly wide variety of fibers (see paragraphs0039-40), and in particular the use of high-strength aramid fibers suchas KEVLAR® fibers, in which the chain molecules in the fibers are highlyoriented along the fiber axis so the strength of the chemical bond canbe exploited.

While Meng's use of a solid dispersion agent may improve dispersion, andthus improve the material properties of the fiber-reinforced rubbercompound generally, Meng does not address the problem of anisotropy inPDM stator manufacturing. As a result, Meng estimates a fiber loadingfor the stator that is too low for optimum performance in the transversecross-section direction (“against the grain”). As disclosed in paragraph0062 of Meng, low fiber loading is preferred in Meng's composites inorder to render minimal impact on properties other than Modulus.

U.S. Pat. No. 8,944,789 to Butuc et al. (“Butuc”) discloses reinforcinga rubber composite with a variety of “reinforcing agents” includingfiber. Disclosed embodiments in Butuc use aramid fibers such as KEVLAR®fibers. Butuc also discloses use of a “dispersing substance” that is acarrier for the reinforcing agent. Butuc confines its disclosure tosolid dispersing substances that include clay, glass, fumed silica,silicon dioxide, diamond and combinations thereof.

Butuc further discloses use of magnetically-responsive particles to beincluded with the reinforcing agents and dispersion substances. In FIG.4C and associated disclosure, Butuc activates a magnetic source throughthe longitudinal center of the stator during curing of the rubbercomposite, with the goal of causing the magnetically-responsiveparticles to align the reinforcement fibers towards the source. As aresult, the “grain” in such stators is substantially uniformly in thetransverse cross-section direction.

While such magnetically aligned fibers may cause the stator to haveimproved properties (such as Modulus) in the transverse cross-sectiondirection, Butuc's magnetic method leaves several drawbacks that do notaddress or remediate the anisotropy problem identified above in thisapplication, at least in any practical way. First, the magneticalignment method of Butuc simply shifts the anisotropy problem into adifferent plane. After magnetic processing, there is operationalweakness in the stator in the longitudinal direction, which is now“against the grain”. Butuc acknowledges as much in column 13, lines 3-14of its disclosure. Second, the magnetic processing creates an additionalmanufacturing step which will add to the manufacturing cost of thestator. Third, there is no disclosure in Butuc regarding what effect, ifany, the magnetically-responsive particles may have on the materialproperties of the finished stator. Finally, there is no validation inButuc (e.g. via disclosed experimentation or examples) that the magneticalignment process actually produces the transversely-aligned fibers assuggested.

There is therefore a need in the art for a rubber composition for usein, for example, PDM stators, that is engineered to address anisotropyproblems caused by the inevitable fiber alignment seen in thelongitudinal helical direction when injection molding is used inmanufacturing. Advantageously the new rubber composition will use asimple manufacturing solution such as high fiber loading in order togenerate serviceable material properties such as high Modulus in thetransverse cross-section direction (“against the grain”).

While serviceable and highly advantageous in its own right to addressanisotropy problems, high fiber loading may enable yet further benefitsin some applications when short aramid fibers are used in the high fiberloading.

Generally speaking, highly fibrillated aramid fibers are advantageous inapplications where high fiber loading is used to address anisotropy.Highly fibrillated fibers provide increased surface branching, and thushigher fiber surface area. The higher the cumulative fiber surface area,the more fiber reinforcing that becomes available to the mix.

However, more highly fibrillated fibers tend to interlock and, as aresult, form fiber clumps and cause more problems with even fiberdispersion and distribution throughout the mix. It is known to extendmixing times to improve fiber dispersion, but extended mix times arealso known to increase production cost, add mechanical stress to thefinished elastomer, and increase the heat buildup in the batch duringmixing. The increased mechanical stress and/or heat buildup leads toadverse effects on the compound during manufacture, such as molecularcleavage and premature scorching.

Another method used to improve fiber dispersion is disclosed in U.S.Pat. No. 8,944,789 to Butuc, as described above. Butuc teaches use of adispersing substance such as clay, glass, fumed silica, silicon dioxide,or diamond. A disadvantage of using such dispersing substances is thatthey introduce an extraneous component to the compound that mayadversely affect physical properties. For instance, fumed silica andsilicon dioxide are known to absorb water and thus increase the tendencyof the finished rubber compounds to swell when exposed to water.

Still another method used to improve fiber dispersion is to pre-dispersethe fiber in liquid dispersion agents (such those as disclosed in U.S.Published Patent Application 2015/0022051 to Meng et al.). Other methodsare known to pre-disperse fiber in a low molecular weight oil and/orelastomer. Such pre-dispersion agents are conventionally mixed as amasterbatch. The masterbatch is then added at selected points throughthe manufacturing steps of the mix. Attempts to improved distribution offibers in the mix via pre-dispersion agents thus have the disadvantageof reducing manufacturing economy, since additional masterbatch stepsare required. Further, adding the pre-dispersion agent to the rubbercompound may adversely affect in-service properties of the finalcompound, such as retention of mechanical properties at elevatedtemperatures.

There is therefore also a need in the art for a technique to improvedispersion of highly fibrillated aramid fibers in rubber compounds withhigh fiber loading. Advantageously, such a technique will not rely onknown methods to promote dispersion, such as use of solid or liquiddispersion agents in the fiber/rubber mix.

SUMMARY AND TECHNICAL ADVANTAGES

These and other drawbacks in the prior art are addressed byfiber-reinforced rubber composites (and methods of manufacture thereof)that are specifically engineered and optimized to provide the desiredmaterial properties. This disclosure describes such optimized rubbercomposites in accordance with two related inventive aspects: (1) highfiber loaded composites generally; and (2) the use of highly fibrillatedaramids blended with short-length, low-fibrillation aramids in such highfiber loaded composites.

In a first aspect (the “First Aspect”), the optimized products maximizethe fiber loading to improve crack resistance and fatigue life whilestill providing a serviceably high Modulus in the transversecross-section direction (“against the grain”) for maximum power outputand longevity.

Conventional fibers may be selected for embodiments according to theFirst Aspect, and are disclosed in U.S. Pat. No. 6,358,171 to Whitfield,for example. The following disclosure is adapted from Whitfield'sdiscussion of conventional fibers.

The type of fibers that may beneficially be used as a reinforcement ofthe stator elastomer include meta-aramids, para-aramids, polyester,polyamide, cotton, rayon and glass, as well as combinations of two ormore of the foregoing, but is preferably para-aramid. The fibers may befibrillated or pulped, as is well known in the art, where possible for agiven fiber type, to increase their surface area, or they may be choppedor in the form of a staple fiber, as is similarly well known in the art.For purposes of the present disclosure, the terms “fibrillated” and“pulped” shall be used interchangeably to indicate this knowncharacteristic, and the terms, “chopped” or “staple” will be usedinterchangeably to indicate the distinct, known characteristic. Thefibers preferably have a length from about 0.1 to about 10 mm. Thefibers may optionally be treated as desired based in part on the fibertype to improve their adhesion to the elastomer. An example of a fibertreatment is any suitable Resorcinol Formaldehyde Latex (RFL).

In embodiments in which the fibers are of the staple or chopped variety,the fibers may be formed of a polyamide, rayon or glass, and have anaspect ratio or “L/D” (ratio of fiber length to diameter) preferablyequal to 10 or greater. In addition, the fibers preferably have a lengthfrom about 0.1 to about 5 mm.

In other embodiments in which the fibers are of the pulped orfibrillated variety, the fibers are preferably formed of para-aramid,and possess a specific surface area of from about 1 m²/g to about 15m²/g, more preferably of about 3 m²/g to about 12 m²/g, most preferablyfrom about 6 m²/g to about 8 m²/g; and/or an average fiber length offrom about 0.1 mm to about 5.0 mm, more preferably of from about 0.3 mmto about 3.5 mm, and most preferably of from about 0.5 mm to about 2.0mm.

The amount of fiber (and preferably, para-aramid fibrillated fiber) usedas reinforcement may beneficially be in a fiber loading from about 1.0to about 20.0 parts per hundred weight of nitrile rubber (or “phr”); ispreferably from about 1.5 to about 10.0 parts per hundred weight ofnitrile rubber, more preferably from about 2.0 to about 5.0 parts perhundred weight of nitrile rubber, and is most preferably from about 2.0to about 4.0 parts per hundred weight of nitrile rubber. One skilled inthe relevant art would recognize that at higher fiber loadingconcentrations, the elastomer would preferably be modified to includeadditional materials, e.g. plasticizers, to prevent excessive hardnessof the cured elastomer or reduced content of carbon black and otherfiller materials.

The fibers may be added to the elastomer composition via any suitableand/or conventional technique, such as by first incorporatingfibrillated fibers in a suitable first elastomer composition to form afiber-loaded Masterbatch having a final fiber content of about 50% byweight, or any other suitable amount; thereafter adding the fiber loadedmaster batch to the stator elastomer composition in order to allow forsuitable distribution of the fiber in the final stator elastomercomposition; and then forming the stator with the thus fiber loadedelastomer composition via any suitable and/or conventional technique.

The nitrile group-containing copolymer rubber composition useful in someembodiments may be cured with sulfur, organic peroxide, or otherfree-radical promoting material. The elastomeric material may also becured in a mixed cure system, utilizing a combination of sulfur, anorganic peroxide or other free-radical promoting material. In disclosedembodiments, the nitrile group-containing, nitrile copolymer rubber issulfur cured. Possible sulfur donors for curing include but are notlimited to tetra-methyl-thiuram di-sulfide, tetra-ethyl-thiuramdi-sulfide, di-pentamethylene thiuram di-sulfide, di-pentamethylenethiuram tetra-sulfide, di-pentamethylene thiuram hexa-sulfide,di-thio-di-morpholine, di-thio-di-caprolactam and 2-(4-morpholinyldi-thio)-benzothiazole. It is believed that if the nitrile rubber iscured with an organic peroxide and reinforced with fiber in accordancewith disclosed embodiments, the high temperature resistance of thestator rubber lining would be even higher than a similar sulfur-curedrubber, and would potentially reach peak operating temperatures of 160°to 165° C. or higher.

Other conventional elastomeric additives, process and extender oils,antioxidants, waxes, pigments, plasticizers, softeners and the like maybe added, in accordance with conventional rubber processing practice.For example, the elastomeric material may also contains 60-90 phr N774carbon black, a plasticizer preferably in an amount up to about 20 partsper hundred weight of elastomer, antioxidants, cure accelerators and acure retarder.

In a second aspect (the “Second Aspect”), embodiments of the disclosedfiber-reinforced rubber composites include manufacturing steps in whichhighly fibrillated aramid fibers are blended with short-lengthlow-fibrillation fibers in the fiber loading. Preferably, theshort-length fibers may be pre-cut from longer low-fibrillation fibers,although this disclosure is not limited in this regard.

The short fibers in the blend help prevent the highly fibrillated fibersfrom “interlocking” with other fibers, thereby encouraging improveddispersion of the blend throughout the rubber mix. In preferredembodiments, the blend also includes no extraneous components to assistfiber dispersion (such as solid or liquid dispersion agents as used inthe prior art) that might adversely affect the properties of the finalcompound. The presence of the short fibers in the blend also obviates aneed for increased mixing times or increased mixing temperatures to aidin fiber dispersion.

Preferably, the short fiber is added by itself directly to the firstpass mix of highly fibrillated fiber and rubber in manufacturing adesired rubber compound, although the scope of this disclosure is notlimited in this regard. The short fibers in the blend interrupt thetendency of the highly fibrillated fibers to clump and “interlock”. Theshort fibers and highly fibrillated fibers are chemically similar, andso the presence of the short fibers in the blend is not a “foreignsubstance” (such as a solid or liquid dispersion agent as known in theprior art), and so does not run the risk of negatively affecting theimproved mechanical properties provided by the highly fibrillatedfibers. In fact, the presence of the short fibers in the blend tends toboost the ability of the highly fibrillated fibers to enhance thematerial properties of the rubber mix, since the highly fibrillatedfibers are distributed more evenly throughout the mix.

Embodiments as described in this disclosure therefore provide a statorfor use in a positive displacement motor, the stator comprising: arubber compound formed into a generally cylindrical rubber stator tubehaving interior helical pathways therein, the helical pathways extendingin a longitudinal direction along the stator tube, the helical pathwaysproviding stator lobes formed in the stator tube in a transversedirection orthogonal to the longitudinal direction; the rubber compoundincluding fiber reinforcement, the fiber reinforcement including aplurality of elongated fibers, the fiber reinforcement having anisotropyin the stator tube, wherein elongated fibers in the fiber reinforcementcreate a grain direction, the grain direction generally with the grainin the longitudinal direction of the stator tube and across the grain inthe transverse direction, in which the rubber compound has a 25% tensileModulus that is at least a 10% lower tensile Modulus across the grainthan with the grain; and the fiber reinforcement including a fiberloading of greater than about 2.5 phr of elongate fibers, the fibers(including elongate fibers) having a 100% tensile Modulus of greaterthan about 60 GPa, and the rubber compound having a 25% tensile Modulusof greater than about 400 psi across the grain and a 50% tensile Modulusof greater than about 700 psi across the grain.

In other embodiments, the fiber reinforcement includes a fiber loadingin a range from about 2.5 phr of elongate fibers to about 10.0 phr ofelongate fibers, and more preferably in a range from about 2.5 phr ofelongate fibers to about 6.0 phr of elongate fibers.

In other embodiments, the fibers (including elongate fibers) may have a100% tensile Modulus in a range of about 60 GPa to about 110 GPa, andmore preferably in a range of about 60 GPa to about 75 GPa.

In other embodiments, the rubber reinforcement includes fiberreinforcement comprising a blend of fibrillated fibers and short cut (orshort chop) fibers in a ratio by weight, and in which: (a) the ratio byweight of fibrillated fibers to short cut (or short chop) fibers in theblend is in a range of about 50:1 to about 3:1; (b) the fibrillatedfibers have surface areas in a range of about 2 m²/g to about 20 m²/g;and (c) the short cut (or short chop) fibers have lengths in a range ofabout 0.05 mm to about 3.0 mm.

In other embodiments, the rubber compound is manufactured according to amixing process in which no masterbatch is used and no pre-dispersionagent is added.

In other embodiments, the ratio by weight of fibrillated fibers to shortcut (or short chop) fibers in the blend is in a range of about 20:1 toabout 4:1.

In other embodiments, which the fibrillated fibers have surface areas ina range of about 7 m²/g to about 11 m²/g.

In other embodiments, the short cut (or short chop) fibers have lengthsin a range of about 0.1 mm to about 2.0 mm.

It is therefore a technical advantage of the disclosed technologyaccording to the First Aspect to create fiber-reinforced rubbercompounds for PDM stator applications that include higher fiber loadingsthan have been seen in the prior art. Such higher fiber loadings takeadvantage of fiber grain anisotropy in the stator (resulting from theinjection process) such that the high fiber loading creates substantialimprovement in tensile Modulus in the longitudinal direction (with thegrain), with comparatively moderated loss or unchanged performance inelongation in the transverse direction (across the grain). These effectstranslate into substantial improvement in power capability of thestator, with only moderately changed to unchanged resistance to cyclicfatigue under load from the rotating rotor.

It is a further technical advantage of the disclosed technology tominimize or eliminate the use of silicon dioxide in the composition.Other methods of random chop fiber and pulp fiber additions, rely on theuse of spherical silicon dioxide shapes to aid in the dispersion of thefibers within the rubber mixture. In low levels, less than 5 phr (partsper hundred rubber), spherical silicon dioxide can provide beneficialproperties such as moisture control and rubber reinforcement. Beyondthis amount, silicon dioxide can degrade properties and absorb moremoisture than desirable, leading to degraded material performance.

It is a further technical advantage of the disclosed technology topromote the full beneficial properties of aramid fibers as reinforcementfor rubber compounds used, for example, in applications such as PDMstators. Conventional thinking suggests that the full potential ofaramid fibers cannot be fully achieved without the aid of dispersionagents, and/or without pre-mixing the fiber pulp in a masterbatch, dueto the inherent tendency of highly-fibrillated aramids to clump whenmixed directly with rubber without such dispersion aids. This disclosureruns counter to such conventional thinking, and in its Second Aspect,seeks to remove low-viscosity substances such as pre-dispersion agentsfrom high-fiber mixes, and further obviate the need for masterbatches.Rubber compounds according the Second Aspect do not contain dispersionagents whose presence in the final product might detract from theelastomeric performance of the rubber compound in service. Fiber blendembodiments according the Second Aspect obviate the need for separatenon-fiber dispersion aids such as silica or silicon dioxide (whether insolid or liquid form). Such embodiments further promote manufacturingeconomy by obviating the need for pre-dispersion steps or a masterbatch,as are common in the prior art. For purposes of this disclosure, amasterbatch is a pre-mixture of elastomer and an active ingredient offormulated compound, such as aramid pulp, that is combined and mixed ina separate step or process prior to being added to a subsequent mixingstep of the desired compound. The masterbatch pre-mixes the activeingredient with another component such as a low molecular weightelastomer to promote efficient mixing in subsequent steps of thecompounding process. Such ingredients as anti-oxidants, reinforcingagents, and curatives are commonly prepared in masterbatches. The termsmasterbatch and pre-dispersion are generally used interchangeably in theart, although a pre-dispersion may sometimes be distinguished to includesolid particles instead of a liquid or elastomer.

A further technical advantage of the disclosed technology is thatembodiments including blends of highly fibrillated fibers andshort-length low-fibrillation fibers facilitate higher loadings of fiberthan would otherwise normally be achieved. The blend including shortfibers improves overall fiber distribution throughout the mix, therebyenabling mixes with higher fiber loadings to be manufactured more easilyand cost-effectively. As discussed in the Background section of thisdisclosure, reinforced rubber mixes engineered to have higher fiberloadings will tend to show improved crack propagation resistance,increasing fatigue life of the rubber in service. When cracks in statorrubber inevitably form under operational cyclic loads, especially in atransverse direction (across the grain) from compression of lobes via arotating rotor, higher levels of fiber loading will inhibit the growthof those cracks because a crack is more likely to encounter a fiber inits propagation (the presence of the fiber arresting further growth ofthe crack). The disclosed blends of highly fibrillated aramids withshort-length, low-fibrillation fibers further enable higher fiberloadings than used conventionally. Such higher fiber loadings in turngenerate advantageous material properties in the finished rubbercompound, such as high Modulus in the transverse cross-section direction(“against the grain”) to prevent crack propagation and to improvefatigue life, and even higher Modulus in the longitudinal direction(“with the grain”) for increased power. Embodiments of the disclosedtechnology may also vary the ratios of highly fibrillated fibers toshort-length low-fibrillation fibers to achieve more precise desiredmaterial properties in the finished compound.

A further technical advantage of embodiments including embodimentsincluding blends of highly fibrillated fibers and short-lengthlow-fibrillation fibers is that shear agents may also be obviated in themixing process. Limitations with aramid fibers such as Twaron® orKEVLAR® pulp by themselves have been the inability to obtain aconsistent mix of the fibers in the bulk materials without either addingmore mechanical shear in mixing or using dispersing agent such assilica, or low molecular weight rubbers and oils. Increased sheardegrades the rubber compound, and, as noted above in this disclosure,dispersion agents add extraneous components that adversely affectcompound properties.

The foregoing has outlined rather broadly some of the features andtechnical advantages of the disclosed rubber reinforcement technology,in order that the detailed description that follows may be betterunderstood. Additional features and advantages of the disclosedtechnology may be described. It should be appreciated by those skilledin the art that the conception and the specific embodiments disclosedmay be readily utilized as a basis for modifying or designing otherstructures for carrying out the same inventive purposes of the disclosedtechnology, and that these equivalent constructions do not depart fromthe spirit and scope of the technology as described and as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments described in thisdisclosure, and their advantages, reference is made to the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1 through 14 illustrate a First Aspect of the disclosedtechnology, and FIGS. 15 through 21 illustrate a Second Aspect of thedisclosed technology, and in which further:

FIG. 1 illustrates anisotropic material behavior of an injected stator,comparing 2.5 phr fiber loading of rubber to unloaded rubber;

FIG. 2 illustrates elongation at break (expressed as a percentage) offiber-loaded rubbers by grain direction;

FIG. 3 illustrates optimum material properties for three rubbercompositions serviceable in a PDM stator;

FIG. 4 is a reference table of Modulus of select fibers compared to thecorresponding Modulus of other well-known materials;

FIG. 5 illustrates a comparison of Modulus performance between polyesterfiber loading and Twaron® fiber loading with the grain;

FIG. 6 illustrates a comparison of Modulus performance between polyesterfiber loading and Twaron® fiber loading against the grain;

FIG. 7 depicts a schematic comparison between the fiber alignment angleand the helix angle in a PDM stator after rubber injection;

FIG. 8 depicts a mathematical formula for calculating the helix angle ina PDM stator;

FIG. 9 is a schematic diagram of conventional specimen testing accordingto grain direction;

FIGS. 10 through 14 depict flow charts illustrating embodiments addingadditional dispersion steps to the disclosure of Examples A1 and A2below;

FIG. 15 illustrates anisotropic material behavior of an injected stator,comparing 4 phr fiber loading of rubber to unloaded rubber, where thefiber loading is according to the Second Aspect disclosed herein;

FIG. 16 depicts elongation change with grain direction in twofiber-loaded rubbers as compared to unloaded rubber;

FIG. 17 depicts currently-preferred optimum properties for rubbercompositions in a mud motor, including optimum properties for acomposition using fibrillated aramid fiber blended with short cut aramidfibers according to the Second Aspect disclosed herein;

FIGS. 18 and 19 are optical microscope views of fiber dispersion in twocomparative samples;

FIG. 20 depicts material properties of comparative rubber compoundsamples after exposure to diesel, brine and water; and

FIG. 21 is a comparison of material properties of a rubber compound madeaccording to Example B1 in this disclosure versus those of a highdurometer NBR compound.

DETAILED DESCRIPTION

First Aspect of this Disclosure

As discussed summarily above in this application, fibers introduced toreinforce a rubber composite will, during injection molding of a PDMstator, align substantially with the flow direction of the helical pathof the lobe geometry. The most significant changes in material physicalproperties will be aligned with this helical path and substantiallyalong the length of the stator (i.e., in the longitudinal direction). Inthe transverse cross-section direction, the material properties willchange much less. FIG. 1 depicts anisotropic material behavior of aninjected stator, comparing 2.5 phr fiber loading of rubber to unloadedrubber. The y-axis units in FIG. 1 are psi. FIG. 1 shows that for a 2.5phr fiber loading as compared to an unloaded (“unfilled”) rubber, theincrease in Modulus in the transverse cross-section direction (“XGRAIN”,or against the grain) is measurably less than the corresponding increasein the helical/longitudinal direction (“WGRAIN”, or with the grain).Further, for 2.5 phr fiber loading, FIG. 1 shows that 25% tensileModulus is greater than about 400 psi against the grain, and that 50%tensile Modulus is greater than about 700 psi against the grain. FIG. 1further shows that at 25% tensile Modulus, the value across the grain isdepicted at about 450 psi, which is at least 10% lower than thecorresponding value with the grain (depicted at about 700 psi). Thiscombination of (1) anisotropic material behavior in stators with highfiber loads, and (2) serviceable tensile Modulus against the grain insuch stators notwithstanding the anisotropic material behavior,distinguishes stators according to the First Aspect of this disclosureover conventional stators.

Performance in the transverse cross-section direction is important in astator, since the performance of a rubber composite in this directionwill often define the performance of a PDM. The fit of the rotor withinthe stator is a primary concern since “fit” dictates power output anddurability of the PDM. If the fit is too tight, the elastomer willprematurely fail from heating associated with excessive contact stressand sliding as well as hysteretic flexing of the rubber lobes. If thefit is too loose, the power output will be low and the motor will tendto stall and/or fail from dynamic overloading of the rubber in thelobes.

As can be inferred from the data shown in FIG. 1 , due to theanisotropic nature of the final manufactured stator, the stiffness of arubber compound with greater than 1.5 phr of fiber will have at least10% lower Modulus in the transverse cross-section direction (“againstthe grain”) than in the longitudinal direction (“with the grain”). Forexample, FIG. 1 shows that the rubber compound has a 25% tensile Modulusthat is more than 10% lower across the grain than with the grain.

The stator as manufactured, with anisotropy, will also have increasedelongation in the transverse cross-section direction. FIG. 2 depictselongation at break (expressed as a percentage) of fiber-loaded rubbersby grain direction. As shown on FIG. 2 , even though the transversecross-section (“XGRAIN”) is known to be less reinforced, thefiber-reinforced rubber composite has much higher elongation in thetransverse cross-section direction than prior art testing and disclosurewould suggest on material assumed to be isotropic. Prior art disclosureshave overlooked the effects of anisotropy, and confined themselves toperforming tests on homogenous mixtures of randomly oriented fibers. Infact, the anisotropy imparted by the manufacturing process of a statorallows a higher fiber loading than is suggested in the prior art,enabling in turn an improvement of material properties such as Modulusin the transverse cross-section direction.

As noted earlier in this application, the highest Modulus will beexperienced in the longitudinal direction of the stator. In thisdirection, the high alignment of fibers is acceptable and providesadditional wear resistance to the erosive effect of the solid particlesfound in drilling mud. When aligned longitudinally, the fibers continueto act as a crack arrestor and minimize failures due to chunking of theelastomer. However, high fiber loading will also give serviceably highModulus in the transverse cross-section direction.

As noted, this application presents a rubber composition with highModulus and optimum elongation in the transverse cross-section directionin order to maximize the overall performance and durability of the PDMin which the stator is deployed. Material properties of the rubbercomposite are optimized when they minimize lobe deflection under highload but also deflect far enough to allow solid mud contaminants to passwithout damaging the elastomer bond or the rotor surface coating.

FIG. 3 depicts optimum material properties for three rubber compositionsserviceable in a PDM stator. This application discloses afiber-reinforced rubber composite that balances the material propertiesillustrated in FIG. 3 for three types of optimized composites.Embodiments preferably comprise fiber loading with greater than about1.5 phr of aramid fibers such as Twaron® or KEVLAR® fibers, with fiberstiffness (100% tensile Modulus) ranging from about 60 GPa to about 110GPa. Ideal stiffening of the unreinforced rubber occurs in this range.More preferably, embodiments comprise fiber loading in the range ofabout 1.8 phr to about 5.0 phr using slightly less stiff aramid fibers,with stiffness (100% tensile Modulus) in the range of about 60 GPa toabout 75 GPa.

Comparing the high fiber loading in the above embodiments to the priorart discussed in the Background section of this disclosure, the abovehigh fiber embodiments improve significantly the rubber composite'spropensity for crack propagation and growth under cyclic operationalloads as compared to, for example, the lower fiber loading disclosed inMeng. Looking at Meng in detail, Meng discloses the use of high Modulusaramid fiber in a rubber composition that has an elongation of 300% witha carbon black loading of 90 phr. By contrast, embodiments of the rubbercomposites disclosed in this application may be loaded with about 2.0phr to about 3.0 phr fiber when the carbon black loading is dropped toabout 75 phr to about 85 phr. This combination of fiber and carbon blackloading will still achieve a target elongation of the base elastomer(i.e. without fiber loading) of greater than about 300% for applicationsspecifically designed for high stiffness and high power output, andgreater than about 400% elongation for applications specificallydesigned for handling high solid contents in the drilling mud. Theseperformance criteria are in distinction to Meng's disclosure, whichfocuses on highly stiff compounds optimized for lower fiber loading.Contrary to Meng, embodiments of the rubber composite disclosed in thisapplication comprise a much higher fiber loading, which is considered tobe much more important in improving the fatigue life and hightemperature durability of the rubber composite.

It will be appreciated that embodiments of the high fiber-loaded rubbercomposites disclosed in this application are not limited to high Modulusaramid fibers such as KEVLAR® and Twaron®. Other embodiments comprisehigh fiber loading of lower modulus Modulus fibers. FIG. 4 is areference table of Modulus of select fibers compared to thecorresponding Modulus of other well-known materials. It will beunderstood from FIG. 4 that fibers such as polybenzimidazole (“PBI”),Nylon 66, and polyester can be used at even higher phr loadings due totheir lower tensile moduli. For example, polyester has a tensile Modulusof about 14 GPa compared to Twarong's 70 GPa (or thereabouts). FIGS. 5and 6 depict comparisons of Modulus performance between polyester fiberloading and Twaron® fiber loading, with the grain on FIG. 5 and againstthe grain on FIG. 6 . FIGS. 5 and 6 show that at a loading of about 2.5phr, a rubber composite loaded with polyester will produce an elastomerwith a 100% tensile Modulus of about 900 psi with the grain while thesame composition reinforced with Twaron® will produce a 100% tensileModulus of about 1050 psi with the grain.

More specifically, embodiments of high fiber-loaded rubber compositesdisclosed in this application include rubber composites loaded withlower Modulus fibers greater than about 1.8 phr, and preferably fromabout 2.0 phr to about 3.0 phr. PBI is also a very good candidate for areinforcing fiber. With a high melting temperature and low tensileModulus of about 6.0 GPa, PBI can be used as a reinforcing fiber in veryhigh loading concentrations, offering significant ability to prevent andarrest cracking, and to distribute loads across the entire compositerubber stator.

It was noted earlier in this disclosure that fibers introduced toreinforce a rubber composite will, during injection molding of a PDMstator, align substantially with the flow direction of the helical pathof the lobe geometry. It was further noted that the most significantchanges in material physical properties will be aligned with thishelical path and substantially along the length of the stator (i.e., inthe longitudinal direction). Following on from this principle, it willbe understood that the orientation of the fiber will not be purelylateral, but will be heavily influenced by the helical angle of thestator lobes. FIG. 7 depicts a schematic comparison between the fiberalignment angle and the helix angle in a PDM stator after rubberinjection. FIG. 8 depicts, purely for reference, a mathematical formulafor calculating the helix angle in a PDM stator. As illustrated on FIGS.7 and 8 , for most models, when using a standard ASTM tensile sheet toapproximate the anisotropy, the angle of maximum anisotropy will beslightly less than the helix angle of the lobe design.

As the injection of rubber flows through the stator mold, the path ofleast resistance occurs in the large cross section of the lobe area inconventionally formed stator tubes. There is a minor amount of crossflow through the thin cross section at the stator valley that works toreduce the fiber and rubber grain alignment with the helix angle of thestator tube but the alignment is found to be substantially along thedirection of the helix geometry. When predicting and optimizing thestator rubber material, laboratory tests are needed to measure thematerial properties. These tests are normally performed in accordance tostandardized ASTM rubber test methods. In order to estimate theproperties of rubber with fiber alignment, fiber can be mixed into therubber and then prepared on rubber processing equipment to simulate theinjection forces working to align the fiber during the manufacturingprocess. One method is to use a standard two roll mill and work therubber in a single direction. The mill will reduce the dimensions of therubber to a flat sheet. When the flat sheet is retrieved from the millrun, the sheet will be rolled and folded to maintain a single fiberdirection. The rubber may be passed through the mill a sufficient numberof times in order to establish the desired degree of alignment. This mayrequire five to fifty passes through the mill.

Once the fiber is aligned, it is desirable to test the materialproperties in critical directions. In the first test, with grain andagainst grain physical properties are established by orienting thecoupons with the rolling direction (“with the grain”) and 90° to therolling direction (“across the grain”) to establish the maximum degreeof anisotropic behavior for the material. FIG. 9 is a schematic diagramdepicting conventional specimen testing according to grain direction.With reference to FIG. 9 , when considering the design function of thestator, a second test is needed to determine the physical properties ofthe stator in the longitudinal and transverse cross section directions.It is the material properties in the transverse cross section directionthat are critical in determining the interference fit stresses at therotor-stator interface of the power section. These properties determinea significant portion of the sealing, friction and hysteretic heatingbehavior of the design.

When considering PDM stators with more uniform rubber cross section(such as are commonly found in “evenwall” stators), the flow of therubber is still dictated by the helix angle of the stator mold geometry.However, in some cases, a higher degree of cross flow is seen. In thesecases the material properties of the transverse cross section can bebetter estimated by using an angle that is 5 to 10 degrees less than thehelix angle of the stator mold design.

Two examples now follow setting forth recipes and mixing processes thatwill yield embodiments of the high fiber-loaded reinforced rubbercomposite disclosed in this application.

EXAMPLE A1 2-Pass Mixing Formulation, Using Commercially Available FiberWith Pre-Dispersion in Rubber

Description PHR First Pass NBR 100 Rhenogran ® P91-40/NBR (solid aramidfiber pre-encapsulated in rubber) 6.25 N774 Carbon Black 80 Antioxidants3 Plasticizers and Processing Aids 20 Tackifier resin 6 Stearic acid 0.5Zinc Oxide 3 Second Pass Sulfur 1.5 Scorch decelerators 6.5 Cureaccelerators 1.7 Total Batch Weight 228.45

EXAMPLE A2 2-Step Masterbatch of Twaron® Pulp Fiber Added to First PassMixing of Fiber-Filled Compound

Description Phr First Step Masterbatch Cray Valley Ricon ® 153 100Twaron ® Pulp (native aramid fiber) 50 Second Step Masterbatch NBR 100First Pass Mixing NBR 100 Twaron ® Dispersion Masterbatch 12.5 N774Carbon Black 80 Antioxidants 3 Plasticizers and Processing Aids 20Tackifier resin 6 Stearic acid 0.5 Zinc Oxide 3 Second Pass MixingSulfur 1.5 Scorch decelerators 6.5 Cure accelerators 1.7 Total CompoundBatch Weight 234.7

The above two examples are currently preferred embodiments at the timeof filing the provisional application to which this application claimspriority, although nothing in this disclosure should be construed tolimit its scope to these two embodiments. Further, at the time of filingof the provisional application, the recipe of Example A1 was (and stillis) preferred of the two disclosed embodiments. However, it will beunderstood that this preference is solely a matter of design choicebased on what is known at the time of filing this application and itspredecessor provisional application.

A primary distinction between Examples A1 and A2 is that in Example A1,Rhenogran® P91-40/NBR (or colloquially “Rhenogran 91”) is used in makingthe Masterbatch. As of the date of filing this application, Rhenogran 91is commercially available from Rhein Chemie Rheinau GmbH in Germany. Atleast as of its Rhein Chemie Technical Data Sheet dated Dec. 15, 2014,Rhenogran P91 comprises Twaron® aramid fibers (by Teijin Aramid in TheNetherlands) in highly fibrillated solid pulp form, where the fibers arecoated with NBR already. Rhenogran 91 disperses well in the NBR andother ingredients in the first pass of Example A1.

By contrast, in Example A2, the first stage of the process is apre-dispersion step, in which native Twaron® aramid pulp fibers aremixed with NBR to create a fibrillated dispersion solid. In effect, thispre-dispersion step in Example A2 is creating a customized, engineeredvariant of the commercially-available Rhenogran 91.

For the avoidance of doubt, it will be understood that the highfiber-loaded reinforced rubber composites disclosed in this applicationare not limited to the commercially-available examples of the productsused in embodiments described. It will be understood that equivalentsubstitutions can be made within the scope of the disclosure.

Referring in more detail now first to Example A1, ingredients are mixedin a two-step process using an internal rubber mixer such as a Banburymodel or another equivalent model. The internal mixer can be configuredwith different mixing blades to change the amount of shear force appliedto the rubber mix. Blade types are known as two-wing, four-wing andintermesh designs and increase in shear action in the order listed. Thismixer type is used in both the first and second pass mixing.

In the first pass mix of the first formula, the NBR polymer is added andsufficiently mixed to raise the temperature to create a flowablemixture. The Rhenogran® P91 fiber dispersion is then added to the mixer.Sufficient time is spent mixing the fiber dispersion into the rubbermixture to establish a well dispersed mixture of fibers. During thispart of the procedure, the mixing parameters may be much longer than anormal mixing sequence to distribute the fibers. The carbon black can beadded after or before the fiber to also achieve a well distributed mix.Anti-oxidants, resins, process aids, zinc oxide and stearic acid canalso be added and mixing continues until sufficient time to distributethe ingredients. The mixture is then deployed onto a roll mill,flattened out, allowed to cool to a solid state, and then cut intosheets for later processing in the second (or subsequent) passes. Thissheeted mix is called the rubber formula “Masterbatch”.

In a second pass mixing routine, the Masterbatch from the first pass issized and weighed to fill the mixer with the curative volumes beingadded. The Masterbatch is mixed to a sufficient flowable condition andcuratives are added, brought to an optimal temperature for deployment ona roll mill. Once on the roll mill, the compound is flattened, andprocessed for final sizing and feeding to a strip mill for green rubberpreparation and packaging. The green (uncured) rubber is shipped to thestator manufacturing plant for final manufacture of the stator shape ona rubber injection machine.

Referring now to Example A2, the mix procedure is similar to the mix inExample A1 except that the process begins with aramid fiber (such asTwaron®) mixed first into a liquid polymer processing aid such as Ricon153 by simple blending or stirring and then set aside for use in theinternal mixer. The 100 phr of NBR rubber is then mixed in the internalmixer to a flowable condition and the Twaron®/Ricon153 solution is addedto the internal mixer and allowed to disperse into the NBR. The mixer isoperated a sufficient time to establish and acceptable dispersion offibers. The mixture is then deployed on a roll mill, and formed intousable Masterbatch sheets of Twaron® Fiber NBR dispersion.

In the second pass of Example A2, the internal mixer is used to bringNBR rubber to a flowable condition and then a portion of Twaron® FiberNBR Masterbatch is added to the mixer. This is allowed to mix untilsufficient fiber dispersion is achieved. After this step, the mixingsequence takes place as disclosed with respect to Example A1.

The following disclosure provides more detail on the mixing processesused in creating compounds with the parameters described above withreference to Examples A1 and A2. In order to disperse small fibers intothe stator rubber composition, it is important to start with a firstmixing procedure where fiber and an elastomeric mix are combined with ahigh percentage of fiber. As disclosed above, advantageously the percentfiber volume is in a range of about 15% to about 80% of the mix, and acombination of elastomer and reactant additives accounts for theremaining proportions of the mix. The ideal mixing procedure is asolution mix method that effectively “functionalizes” the fibers with arelatively uniform coating of elastomer. This is best accomplished whenfiber volumes fall within the optimum range of about 15% to about 80%.

Solution mixing processes for embodiments set forth in this disclosureare similar to those found in U.S. Pat. No. 5,391,623 to Frances andU.S. Pat. No. 6,160,039 to Kleinknecht. The '623 patent to Francesteaches a solvent mixing procedure where elastomer is dissolved insolvent to make a elastomer rich mixture to which aramid fiber is added.The solvent is then dried while the mixture is continuously agitated.

The '039 patent to Kleinknecht describes a slightly different processwhere a water based latex solution is used to coat fiber. In thisprocess the fiber is added to a base elastomer composition that may befurther reacted to form additional elastomer polymerization.Polymerization additives may be present in the latex solution and arelater activated after removal of the water from the solution. During asecond solution preparation, the additives are activated to enhance thepolymerization sequence. Once final polymerization is complete, thecoated fibers are dried to form a chip or whisker shaped pre-dispersionagent ready for mixing into rubber compositions as admix to internalrubber mixers. In addition to these methods, other solution mixingmethods may be used to obtain the first masterbatch.

More traditional methods may be used where a blender or mixing devicecan be used to add liquid polymer such as those offered by the CrayValley Company under the description of low molecular weightcross-linkable processing aids. As before, mixing may be done in aBanbury internal mixer or similar/equivalent. In the case of directliquid polymer or process oil additions to fiber (as in Example A2), ablender/mixer similar to a Ross Double Planetary mixer is utilized.Examples of suitable polymer include Ricon 153, Ricon 1731, and Ricon1754. These liquid polymers can accept similar fiber loading as theprevious examples and can act as a pre-dispersion agent. In the case ofliquid polymer dispersion, there is no subsequent drying process neededand the dispersion is preferably mixed or blended and immediately addedto the internal mixer containing a higher viscosity elastomer carrier orbase rubber compound elastomer.

The above procedures describe an “admix” mixing procedure. In thisprocess, the mixing and dispersion of each component can be broken intomany pre-dispersion steps that produce what is referred to amasterbatch. Each masterbatch is added to components of the rubber mixin a sequence that maximized dispersion of the small particles or fibersin the desired final mixture of curable rubber preform.

It will be further understood that small fibers used as reinforcementaccording to the First Aspect may offer resistance to thoroughdispersion into rubber mixtures, and so pre-dispersion steps arepreferred at the front end of the mixing process. Further embodiments ofthe high fiber-loaded reinforced rubber composites disclosed in thisapplication may require additional dispersion steps. In someembodiments, three to six mixing steps may be needed order to achieveexcellent dispersion of small fibers with high surface area ratio. FIGS.10 through 14 are flow charts illustrating embodiments adding additionaldispersion steps to the disclosure above of Examples A1 and A2. FIGS. 10through 14 are self-explanatory. FIG. 10 is an exemplary three-stepmixing procedure. FIG. 11 is an exemplary four-step mixing procedureincluding a low viscosity polymer additive in the second step. FIG. 12is an exemplary four-step mixing procedure including a medium to highviscosity polymer in the second step. FIG. 13 is an exemplary 5-stepmixing procedure. FIG. 14 is an exemplary 6-step mixing procedure.

Second Aspect of this Disclosure

As also discussed above in this disclosure with respect to the FirstAspect, fibers introduced to reinforce a rubber composite will, duringinjection molding of a PDM stator, align substantially with the flowdirection of the helical path of the lobe geometry. The most significantchanges in material physical properties will be aligned generally withthis helical path and substantially along the length of the stator(i.e., in the longitudinal direction). In the transverse cross-sectiondirection, the material properties will change much less. As a result,anisotropy occurs between longitudinal and transverse directions. FIG.15 is analogous to FIG. 1 , discussed above with respect to the FirstAspect. However, in contrast to FIG. 1 , FIG. 15 depicts Modulusanisotropy in an injected stator, comparing 4 phr fiber loading tounloaded (“unfilled”) rubber, where fiber loading comprises aramid fiberblends according to the Second Aspect. FIG. 15 should be viewed withparticular reference to the trends shown on FIG. 15 rather thanempirical units and absolute values shown on FIG. 15 . With reference toFIG. 15 , while the fiber loading increases the Modulus (PSI, y-axis)over an unloaded rubber, the increase in Modulus in the transversecross-section direction (“XGRAIN”, or against the grain) is measurablyless than the corresponding increase in the helical/longitudinaldirection (“WGRAIN”, or with the grain). However, the Modulus in thetransverse direction (“XGRAIN”) is still substantially higher than isexhibited by the base unfilled elastomer.

As described in more detail above, generally in association with FIG. 1, performance in the transverse cross-section direction is important ina stator, since the performance of a rubber composite in this directionwill often define the performance of a PDM. As can be inferred from thetrends shown in FIG. 15 , due to the anisotropic nature of the finalmanufactured stator, the stiffness of a rubber compound with 4 phr offiber will have at least 10% lower Modulus in the transversecross-section direction (“against the grain”) than in the longitudinaldirection (“with the grain”).

The stator as manufactured, with anisotropy, will also have increasedelongation in the transverse cross-section direction as shown below inFIG. 16 (Elongation, % at break, y-axis). FIG. 16 depicts elongationchange with grain direction for two fiber-loaded rubbers as compared tounloaded rubber. With reference to FIG. 16 , although elongation “withthe grain” is less than an unloaded rubber, elongation “against thegrain” is similar at 3 phr fiber and 4 phr fiber to the unloaded rubber.FIG. 16 further illustrates that even though the transversecross-section (“against the grain”) is known to be less reinforced thanin the longitudinal direction (“with the grain”), the fiber-reinforcedrubber composite has much higher elongation in the transversecross-section direction than prior art testing and disclosure wouldsuggest on material assumed to be isotropic.

As used herein, the terms “fibrillated fiber” and “aramid pulp” are usedinterchangeably. The terms “fibrillated” and “pulped” are also usedinterchangeably, as are the terms “cut”, “chopped” and “staple”.

As discussed summarily above in the Summary section, rubber compositesaccording to the Second Aspect are high fiber loaded, where the fiber isa blend of highly fibrillated fibers and short-length, low-fibrillation(“short cut” or “short chop”) fibers. In currently preferredembodiments, the fibers used in the fibrillated fiber/short cut fiberblend are aramids. For purposes of this disclosure, a highly fibrillatedfiber is a chopped fiber strand whose surface has been mechanicallyabraded to create micro- or nano-sized fibers attached to the main fiberstrand. The fibrillated fiber thus has a very high surface area ascompared to an unfibrillated fiber strand. Such fibrillated fiber as awhole takes the form of an aramid pulp. By contrast, short cut fibersare fiber pieces that are cut or chopped down in length from theoriginal fiber strand. Short cut fibers typically have minimalfibrillation. The combination of fibrillated and short cut fibersprovide a superior mix of fibers for effectively creating a rubbercompound for use in PDM stators without adding extraneous components oringredients via a masterbatch or pre-dispersion—extraneous components oringredients that might, as described elsewhere in this disclosure,otherwise compromise the mechanical properties and aging characteristicsof the finished rubber compound.

FIG. 17 depicts currently-preferred optimum properties for rubbercompositions in a mud motor. Properties are shown for a conventional“soft” rubber, a conventional “hard” rubber, and a composition usingfibrillated aramid fiber blended with short cut aramid fibers accordingto the Second Aspect of this disclosure. As will be seen from FIG. 17 ,rubber compositions according to the Second Aspect balance the materialproperties between conventional “hard” and “soft” rubbers.

Currently preferred embodiments of the Second Aspect comprise fiberloading in a range from about 1.5 phr to about 10.0 phr, where the fiberused for loading is the blend of highly fibrillated fibers and short cutfiber, although the scope of this disclosure is not limited in thisregard. More preferably, embodiments comprise fiber loading in the rangeof about 1.8 phr to about 6.0 phr, although again the scope of thisdisclosure is not limited in this regard.

In currently preferred embodiments according to the Second Aspect, thefibrillated fibers preferably have a surface area from about 2 m²/g toabout 20 m²/g, and more preferably from about 7 m²/g to about 11 m²/g,although the scope of this disclosure is not limited in this regard. Theshort chop fibers blended with the fibrillated fibers preferably have alength of about 0.05 mm to about 3.0 mm, and more preferably from about0.1 mm to about 2.0 mm, although again the scope of this disclosure isnot limited in this regard. The ratio of fibrillated fiber to short chopfiber is preferably in a range from about 50:1 to about 3:1 by weight,more preferably from about 20:1 to about 4:1 by weight, although againthe scope of this disclosure is not limited in this regard. It will beunderstood that in particular, the ratio of fibrillated fiber to shortchop fiber may be varied per user design to achieve optimal propertiesfor the desired application. The reinforcing fibers may optionally betreated as desired to improve their adhesion to the rubber based in parton the fiber type. An example of a fiber treatment is any suitableResorcinol Formaldehyde Latex (RFL).

The following Example B1 illustrates a currently preferred recipe formixing a rubber compound according to the Second Aspect. Example B1loads the rubber compound with about 4 phr aramid fiber, and usesVaramix®, a commercially available aramid pulp from Finite Fiber, aDowco, LLC company, based in Akron, Ohio, U.S.A. Varamix® is a pre-mixedblend of highly fibrillated fibers and short chop fibers with a smallquantity of antistatic agent added. The precise blend may be specifiedby the customer according to the manufacturing application for theblend. The Varamix® blend in Example B1 comprises about 84% fibrillatedfiber by weight, about 14% short chop fiber by weight, and about 1%antistatic agent by weight. The fibrillated fiber in the blend comprisesfibers having surface areas in a range from about 7 m²/g to about 11m²/g. The short chop fiber in the blend comprises fibers having lengthsin a range from about 0.1 mm to about 2.5 mm.

EXAMPLE B1 First Pass Mixing of Varamix® Directly with NBR, withoutDispersion Agent or Masterbatch

Description PHR First Pass - 4 min. Total, Dump below 310 F. NBR 100Varamix ® aramid fiber (mix of long 4 fibrillated fibers with short cutfibers) N754 Carbon Black 79 Antioxidants 3 Plasticizers and ProcessingAids 40 Tackifier resin 6 Stearic acid 1 Zinc Oxide 5 Second Pass - 2min. Total, Dump below 220 F. Sulfur 3 Scorch decelerators 3 Cureaccelerators 3 Total Batch Weight 249

In Example B1, ingredients were mixed in a two-step process using aninternal mixer such as a Banbury or other equivalent model. The internalmixer can be configured with different mixing blades to change theamount of shear force applied to the rubber mix. Tangential mixingblades were used in the recipe above. In the first pass, the polymer andantioxidants were added first and mixed for up to 45 seconds, then aportion of carbon black, all of the fiber, and a portion of plasticizerswere added. At periodic intervals over the mixing cycle, the rest of thecarbon black, plasticizers, zinc oxide, and stearic acid were added.After about 4 minutes the first pass was dumped at a temperature below310° F. and placed on a roll mill at about 120° F. For the second pass,the curatives were added, then swept, then dumped after about 2 minutesat a temperature below 220° F. The final dump was then placed on a rollmill at about 120° F. to form into the final strips.

FIG. 21 is a comparison of material properties of a rubber compound madeaccording to Example B1 versus those of a high durometer NBR compound.On FIG. 21 , the Example B1 compound shows significant improvement in25% and 100% tensile Modulus in the direction of the grain with onlyminimal corresponding change in elongation against the grain, ascompared to a high durometer NBR compound. When deployed in a PDMstator, the large increase in Modulus in the longitudinal direction(“with the grain”) seen in the Example B1 compound should translate intosignificant improvement in power output, while the minimal change inelongation against the grain should translate into unchanged resistanceto cyclic fatigue under load from the rotating rotor.

FIGS. 18 and 19 are optical microscope views of fiber dispersion in twocomparative samples. Two rubber compounds were prepared in a laboratoryscale tangential mixer. The first was prepared with 13 phr of KEVLAR®Merge IF770 aramid in a masterbatch (3 phr effective aramid fiberloading and 10 phr NBR elastomer and other ingredients). The second wasmade according to the Second Aspect disclosed herein, without amasterbatch or any pre-dispersion agents, and comprised 2 phr ofVaramix® per disclosure above. Both compounds were prepared formicroscope analysis by cutting a very thin sample. FIG. 18 shows theKEVLAR® compound under an AmScope optical microscope at 10×magnification, and FIG. 19 shows the Varamix® compound under the samemicroscope at the same magnification. In each of FIGS. 18 and 19 , thelighter colored areas are the fibers as found within the respectivecompounds. In FIG. 18 , a large clump of fiber is clearly visible. Bycontrast, in FIG. 15 , the fibers are much more evenly distributed withlittle or no clumping.

An experiment was also conducted to validate that rubber compositionsmade according to the Second Aspect disclosed herein, with no extraneousingredients included to act as dispersion agents (for example), showimproved performance when exposed to common downhole environments. FIG.20 depicts material properties of comparative rubber compound samplesafter exposure to diesel, brine and water. Two rubber compounds wereprepared in a laboratory scale tangential mixer. The first was preparedwith 13 phr of KEVLAR® Merge IF770 aramid in a masterbatch (3 phreffective aramid fiber loading and 10 phr NBR elastomer and otheringredients). The second was made according to the Second Aspectdisclosed herein, without a masterbatch or any pre-dispersion agents,and comprised 4 phr of Varamix® per disclosure above. Tensile bars weremade of each sample. Some bars were aged in 250° F. diesel, some in 250°F. brine, and some in 250° F. water, all for 72 hours. Theseenvironments were selected because most PDM stators are used withdiesel-, brine-, or water-based drilling muds at temperatures rangingfrom 150° F. to 400° F. After aging, the bars were tested for tensilestrength per ASTM D412. The results were compared with correspondingsample bars that had undergone no aging.

FIG. 20 shows the results. The sample made with a masterbatch includinga dispersion agent displayed over a 20% decrease in 100% tensile 100%Modulus when exposed to diesel or water. By contrast, the “aramid fibersonly” compound (made according to the Second Aspect disclosed herein)displayed less than a 5% change in 100% tensile 100% Modulus whenexposed to diesel, brine, or water. Changes when exposed to brine wereminimal for either sample. The results indicated that the dispersionagent included in the masterbatch sample adversely affected the longterm exposure and aging properties of the sample. Such loss ofmechanical properties in these environments would cause a large loss inperformance with such a masterbatch-based compound deployed in a statorin downhole service. By contrast, a compound made according to theSecond Aspect herein should not experience such a loss in performance.

Although the inventive material in this disclosure has been described indetail along with some of its technical advantages, it will beunderstood that various changes, substitutions and alternations may bemade to the detailed embodiments without departing from the broaderspirit and scope of such inventive material as set forth in thefollowing claims.

We claim:
 1. A rubber compound for use in a stator, the rubber compoundcomprising: a fiber reinforcement, the fiber reinforcement including aplurality of fibers, wherein fibers in the fiber reinforcement create agrain direction in which “with the grain” is generally orthogonal to“across the grain”, wherein the rubber compound has a first value for25% tensile Modulus “across the grain” and a second value for 25%tensile Modulus “with the grain”, wherein the first value is at least10% lower than the second value; wherein the fiber reinforcementincludes a fiber loading of greater than 1.0 phr of fibers; and whereinthe rubber compound has a 25% tensile Modulus of greater than 400 psi“across the grain” and a 50% tensile Modulus of greater than 700 psi“across the grain”.
 2. The rubber compound of claim 1, in which thefiber reinforcement includes the fiber loading in a range from 1.0 phrof fibers to 20.0 phr of fibers.
 3. The rubber compound of claim 1, inwhich the fiber reinforcement includes the fiber loading in a range from1.0 phr of fibers to 6.0 phr of fibers.
 4. The rubber compound of claim1, in which the fibers have a Modulus of greater than 60 GPa.
 5. Therubber compound of claim 1, in which the fibers have a Modulus in arange of 60 GPa to 110 GPa.
 6. The rubber compound of claim 1, in whichthe fibers have a Modulus in a range of 60 GPa to 75 GPa.
 7. The rubbercompound of claim 1, in which the fiber reinforcement comprises a blendof fibrillated fibers and short chop fibers in a ratio by weight, and inwhich: (a) the ratio by weight of fibrillated fibers to short chopfibers in the blend is in a range of 50:1 to 3:1; (b) the fibrillatedfibers have surface areas in a range of 2 m²/g to 20 m²/g; and (c) theshort chop fibers have lengths in a range of 0.05 mm to 3.0 mm.
 8. Therubber compound of claim 7, in which the ratio by weight of fibrillatedfibers to short chop fibers in the blend is in a range of 20:1 to 4:1.9. The rubber compound of claim 7, in which the fibrillated fibers havesurface areas in a range of 7 m²/g to 11 m²/g.
 10. The rubber compoundof claim 7, in which the short chop fibers have lengths in a range of0.1 mm to 2.0 mm.
 11. The rubber compound of claim 1, in which therubber compound is manufactured according to a mixing process in whichno masterbatch is used.
 12. The rubber compound of claim 1, in which therubber compound is manufactured according to a mixing process in whichno pre-dispersion agent is added.
 13. The rubber compound of claim 1, inwhich the rubber compound is manufactured according to a mixing processin which no masterbatch is used and no pre-dispersion agent is added.14. The rubber compound of claim 1, in which the fiber reinforcementcomprises a blend of at least two fiber types, at least one fiber typein the blend having at least one unique characteristic among other fibertypes in the blend wherein said at least one unique characteristic isselected from the group consisting of: (a) length; (b) surface area; (c)chemistry; and (d) Modulus.
 15. A rubber compound for use in a stator,the rubber compound comprising: a fiber reinforcement, the fiberreinforcement including a plurality of fibers, wherein fibers in thefiber reinforcement create a grain direction in which “with the grain”is generally orthogonal to “across the grain”, wherein the rubbercompound has a first value for 25% tensile Modulus “across the grain”and a second value for 25% tensile Modulus “with the grain”, wherein thefirst value is at least 10% lower than the second value; wherein thefiber reinforcement includes a fiber loading of greater than 1.5 phr offibers; and wherein the rubber compound has a 25% tensile Modulus ofgreater than 400 psi “across the grain” and a 50% tensile Modulus ofgreater than 700 psi “across the grain”.
 16. The rubber compound ofclaim 15, in which the fibers have a Modulus of greater than 60 GPa. 17.The rubber compound of claim 15, in which the fiber reinforcementcomprises a blend of fibrillated fibers and short chop fibers in a ratioby weight, and in which: (a) the ratio by weight of fibrillated fibersto short chop fibers in the blend is in a range of 50:1 to 3:1; (b) thefibrillated fibers have surface areas in a range of 2 m²/g to 20 m²/g;and (c) the short chop fibers have lengths in a range of 0.05 mm to 3.0mm.
 18. A rubber compound for use in a stator, the rubber compoundcomprising: a fiber reinforcement, the fiber reinforcement including aplurality of fibers, wherein fibers in the fiber reinforcement create agrain direction in which “with the grain” is generally orthogonal to“across the grain”, wherein the rubber compound has a first value for25% tensile Modulus “across the grain” and a second value for 25%tensile Modulus “with the grain”, wherein the first value is at least10% lower than the second value; wherein the fiber reinforcementincludes a fiber loading of greater than 2.0 phr of fibers; and whereinthe rubber compound has a 25% tensile Modulus of greater than 400 psi“across the grain” and a 50% tensile Modulus of greater than 700 psi“across the grain”.
 19. The rubber compound of claim 18, in which thefibers have a Modulus of greater than 60 GPa.
 20. The rubber compound ofclaim 18, in which the fiber reinforcement comprises a blend offibrillated fibers and short chop fibers in a ratio by weight, and inwhich: (a) the ratio by weight of fibrillated fibers to short chopfibers in the blend is in a range of 50:1 to 3:1; (b) the fibrillatedfibers have surface areas in a range of 2 m2/g to 20 m2/g; and (c) theshort chop fibers have lengths in a range of 0.05 mm to 3.0 mm.